Selenium A look at the intake in different parts of the world and its deficiencies and toxicity by the Merck Manual.

Vitamin and Mineral Deficiencies Provides basic guidelines for when supplements are necessary. Explains when a healthy diet may not be enough, and possible health problems due to dietary deficiencies or overuse of supplements.

Vitamin B12 (Cobalamin) Article describes the function of this vitamin in the body and symptoms, diagnosis, and treatment of deficiency.